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^ APRIL 1996

^AJ

ISSN 0952-7583

Vol. 9,

BRITISH JOURNAL OE

ENTOMOLOGY

AND NATURAL HISTORY

^^^-^OMo^Q

BRITISH JOURNAL OF ENTOMOLOGY AND NATURAL HISTORY

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Wfi.. J. ENT. NAT. i996 ^^^^^^^^^^^^^^^^^^H

LADYBIRD POPULATION EXPLOSIONS

Michael E, N, Majerus and Tamsin M. O. Majerus

Department of Genetics, University of Cambridge, Downing Street, Cambridge CB2 3 EH.

This paper has been prompted by two recurring features of the last ten years,
during which time we have co-ordinated a nation-wide survey of ladybirds in Britain.
One involves the phone calls we receive from members of the media, almost every
year in late July, or early August, asking us to comment on 'plagues of ladybirds'
that have hit such-and-such a place. The second involves the commonest ladybird
reminiscence of members of the public whom we have met while travelling around
Britain either looking for, or talking about ladybirds: ''the year that ladybirds went
mad'\ Almost all are remembering the last 'great ladybird year' — 1976; although a
few also recall the preceding one, in 1959. Much has been written in the press, both
local and national, about plagues of ladybirds, their causes and effects. A
considerable proportion of these column inches is ill-informed. The impression we
often get when talking to journalists is that their aim is to write something that is
worth printing, even if it means making a proverbial mountain out of a molehill. In
our opinion this is something of a shame, as regular and often exaggerated reports of
natural phenomena, which are a little out of the ordinary, undoubtedly devalue
reports of truly extraordinary natural phenomena. Ladybird 'plagues' provide an
excellent example, for major ladybird population explosions are truly amazing
events.

Ladybird plagues, whether widespread or local, are well-known natural
phenomena. Records of swarms are scattered widely through the literature over
the past century. Tn this paper we give an account of some of the reminiscences of the
most recent major ladybird population explosion in Britain, that of 1976, put on
record some of the sightings of ladybird swarms between 1984 and 1995, and
comment on the causes and consequences of ladybird swarms*

The great ladybird year^ — 1976

The population explosion of ladybirds in 1976 was truly astounding. It extended
across more or less the whole of England and Wales, and into some parts of southern
Scotland. Exceptional numbers of ladybirds were also recorded in many parts of
north-west Europe, The huge numbers of ladybirds were widely reported in
newspapers, and on television and radio. In Appendix 1 are listed a small sample of
the many reports and reminiscences of the plagues of ladybirds that year.

Rapid increases in the size of ladybird populations are due to a combination of
factors rather than a single one. Principal among these are food availability, sunshine
during the early summer, temperature during the middle of the summer, and the
mildness of the preceding winter. Other factors, such as the abundance of parasitoids
of ladybirds, also play a role. The way some of these factors fit together may be
understood by considering the prevailing conditions prior to the population
explosion in 1976.

The summers of both 1975 and 1976 were unusually warm and sunn y. Because the
extent of any change in the size of a population of organismsit]j'^9]0s^JjKpQine extent

/ ' '^

e-mail: menm{5)mole. bioxam.ac.uk / MAY 1 Q WQA

BR. J, ENT. NAT HIST, 9: 1996

on the base number of individuals, it is pertinent to start in the summer of 1975. This
was a long warm summer, not exceptionally dry or hot, but with warmer
temperatures than average and no significant long periods of bad weather. Both
aphids, and the ladybirds that fed on them, did well. At least some species of ladybird
such as the 2-spot Adalia bipunctata (L.)^ 7-spot Coccinella septempunctata L. and
10-spot Adalia decempimctata (L.) produced a partial second generation.
Consequently, by the autumn of 1975, the populations of aphidophagous ladybirds
that retired to their overwintering refuges were already larger than usual. The
ensuing winter was remarkably mild, with a result that the winter mortality rate of
ladybirds, which is usually in excess of 50%, was much lower than this figure,
thereby increasing the number of ladybirds in the spring of 1976 still further,
relative to the norm.

The spring of 1976 was not exceptional If anything it was wetter and warmer than
average. Aphid populations did well, reproducing rapidly on the lush plant growth,
so that when ladybirds began to venture out in search of food, they found plentiful
supplies. Cloudy weather in April and early May prevented most species from
indulging in much mating early on, but when the sunny weather began in the middle
of May, conditions were ideal. The ladybirds began to mate and oviposit at a very
rapid rate. Larvae found plentiful supplies of aphids and, as temperatures rose to
record levels, they fed up and completed their development in an abnormally short
time for Britain. The hot sunny weather continued day after day through the rest of
May and throughout June. By the end of June, the extent of the increase in ladybird
numbers, in England, Wales and southern parts of Scotland, was already evident.
They were everywhere. The species that showed the greatest increases were the
aphidophagous generalists, such as the 7-spot, 2-spot and 14-spot Propylea
qnattuordecimpimctata (L,), but most other species did better than usual and 10-
spot, 11 -spot Coccinella undecimpunctata L, and cream-spot ladybird Calvia
quattiiordecimguttata (L.) were more abundant than we have seen them since.

This increase in the number of adult ladybirds, combined with the huge densities
of ladybird larvae among aphid colonies (and high densities of other aphid predators
and parasitoids) led to a dramatic crash in aphid numbers. Bluntly, the aphids were
massacred. Even with their phenomenal reproductive rate, the aphids could not keep
up with the losses due to predation. Worse still the plants that the aphids were
feeding upon were deteriorating rapidly under the scorching heat of that long hot
summer.

By the middle of July, the aphids on the ladybirds' normal host plants had, more
or less, been eaten out. The ladybirds then began to search for food elsewhere. They
took to the air in huge numbers, billions upon billions of them. Some found food on
unusual host plants, but the aphid populations there were soon decimated as well by
the sheer weight of numbers of hungry ladybirds. Day after day, in the second half of
July and on into August, the great swarms of ladybirds took to the air as the
temperatures rose in the mornings. They flew hither and thither, partly being carried
on the prevailing winds, until they came to the coast. Here they stopped, being
brought to earth by the air currents at the coast and possibly by a reticence to cross
wide expanses of water. Vast numbers were reported along all the coasts of England
and Wales, with the south and east coasts being most affected.

The numbers of ladybirds were so legion by the end of July, that they drove
holiday makers from the sea-side resorts. There were many reports of them 'stinging'
or biting humans, although, in truth, all the starving beetles were doing was trying to
find food. In their desperation, they tested the edibility of anything that might have
been nutritious to them. They did not find humans palatable, but their attempts to

BR. J. ENT, NAT. HIST, 9: 1996

test whether we were edible left many people with little bite bumps as our own
chemical defences reacted to the minute droplets of pre-digestive enzyme that
ladybirds inject into their prey when they bite. Untold millions fell into the sea, or
were washed off the coastal sands, muds and rocks as the tide rose and fell. Many
were washed back onto the coast by ensuing tides, so that the tide-line appeared to
comprise little but the corpses of ladybirds.

We once calculated the approximate number of ladybirds that would have been in
the tide-lines along the southern and eastern coasts of Britain on a single day in late
July 1976. Assuming all to have been one of our larger species, the 7-spot (which they
were not), a conservative estimate gives a figure of some 23654400000 ladybirds
(estimated by counting the number of 7-spots in 15 200 mm sections of tideline and
multiplied up by the amount of suitable coastline between Land's End and the east
coast border between England and Scotland). This figure is difficult to comprehend,
but it is about four times the current human population of the Earth and, of course,
this was just the ladybirds in the tide-hnes on a single day. It does not include any of
those that stayed on land, or those that were washed out to sea, or those that were
eaten by other starving ladybirds or other predators, or those that were killed on the
roads or elsewhere by the devices of man. Not surprisingly, a crash in ladybird
populations followed.

From this description, it may be seen that a whole sequence of conditions are
involved in the run up to one of the great ladybird explosions: favourable
conditions the previous year, a mild winter, plenty of food in the spring, a single
and synchronized start to the reproductive season brought on by a sharp
improvement in sunshine hours, and hot and sunny weather in the summer to
promote mating, egg laying and rapid larval development. It is not often that all
these conditions fall together, with the result that widespread ladybird explosions
are rare in Britain, occurring, on average, about once every 15 years. We seem to
be overdue for one now:

Records of ladybird plagues^ in Britain: 1984-1994

During the Cambridge Ladybird Survey, we have received many reports of
abnormal numbers of ladybirds being seen together. A few samples of these records
are given in Appendix 2, together with notes on the observations culled from the
reports sent in by recorders. The records come from two main periods of the year: the
early spring, and what might be termed high summer, covering the last two weeks of
July and the first two weeks of August.

The reasons that large numbers of ladybirds tend to be observed together in these
two periods are different. Many species of ladybird aggregate together to pass the
winter. Large numbers of individuals may find the same sheltered area out of the
worst of the winter weather. When these first venture out from their overwintering
sites they will often do so en masse. On bright spring days, ladybirds will often sun
themselves to warm up. This means that large numbers will be seen in exposed
positions sunning themselves before moving off to forage for food. Most of these
spring records involve the 7-spot ladybird which tends to overwinter close to the
ground, or occasionally underground, often in large aggregations. On leaving its
overwintering sites, this species will first climb low spring vegetation to sun itself, to
drink dew or rain drops to replenish its fluid reserves, and to look for whatever food
it can find. Here, on low-growing green leaves, it is very obvious and easily seen.

It is in high summer that the largest numbers of ladybirds come together and
sometimes form what may realistically be called swarms. At this time of year, the

BR, J, ENT. NAT. HJST,, 9: 1996

adult population usually reaches its maximum. The new generation of aduhs has not
yet begun to diminish because of the perils oflife: lack of food, predators, parasites,
disease, adverse climate or pure accident. The main activity for these young adults is
to feed up for the winter. In many years, aphid populations 'decline dramatically in
high summer, partly as a result of predation pressure and partly because their host
plants are deteriorating. The consequence is that in areas where food has become
scarce, ladybirds will often take to the air on hot summer days, to seek food
elsewhere. The direction of ladybird flight is not entirely active. While ladybirds can
fly in a direct way, when they take to the air on hot days their flight is aided by
thermals. These thermals may take the ladybirds up hundreds, even thousands, of
metres into the sky. At high altitude, they are then blown by the prevailing winds,
often for many miles. Due to the nature of thermals in some places, and particularly
along the coast, large numbers of ihese high-flying fady birds may be brought back to
earth in the same place, producing very high population concentrations. The
prevaihng south-westerly winds are thus responsible for the fact that swarms are
more often recorded from the east coast of England than any other region.

The necessity for synchronized reproduction

Most of the factors that appear to be prerequisites for ladybird population
explosions are easily understood. Good climatic conditions and high food
availability the previous year lead to an increase in the number of ladybirds that
feed up well prior to overwintering. The high fat reserves of these ladybirds, and/or a
mild winter, will result in reduced mortality during this critical period. Conditions
under which aphids flourish in the spring will lead to good suppUes of food for the
ladybirds when I hey leave their overwinter sites and begin feeding and reproducing.
Warm sunny weather in late spring and early summer will induce high mating
frequencies and rapid oviposition. The high aphid density and warm weather will
allow rapid larval development and reduce the level of larval cannibalism. All this is
intuitively sensible. However, the reason why a single and synchronized start to the
ladybird reproductive season is crucial, is not so obvious. It is not easy to see why a
somewhat staggered start to the reproductive season would not produce even greater
numbers if the start of the season were extended earlier- Surely, one might expect that
a staggered start would produce greater numbers because, at least in the early part of
the season when aphid numbers are just building up, survival of larvae would not be
reduced too much by competition between them.

Three factors appear to be important in reducing the benefit of an early and
staggered start to the ladybird reproductive season. First, any significant predation
early in the spring may greatly reduce the eventual numbers of aphids produced
later in the year. Because aphids have such a phenomenal reproductive potential
within a single year, the death of a single female in April may reduce the July
population by many millions. Second, as female ladybirds are reluctant to oviposit
on plants which are already inhabited by ladybird larvae {Hemptinne ef ciL, 1992),
stretching the reproductive season will reduce the number of suitable oviposition
sites for females later in the season. Third, and perhaps most crucially, parasitoids
of ladybirds may benefit from the presence of immature stages over a protracted
period. Monitoring of population demography in 1990 showed the influence that
predators and parasitoids may have in years when the reproductive period for
ladybirds is staggered.

Records in late 1989 indicated that numbers of several of the generalist species of
ladybird were at their highest levels since 1976. The winter of 1989/90 was

BR. J. £NT NAT. HIST,, 9: 1996

exceptionally mild, and winter mortality was low. In East Anglia the 7-spot ladybird
was about 20 times more common in the spring of 1990 than it had been in the spring
of 1989. Similar reports of unusually large numbers of ladybirds came in from most
parts of the country. In May, reports of exceptionally large numbers of aphids were
published in the national press. Conditions appeared ripe for a major ladybird
population explosion. However, it never materialized on a wide scale.

The winter of 1989/1990 was not only mild, it ended early, producing an
abnormally early spring (for the second year running). Ladybirds of many species,
including the aphidophagous generalists, began mating and ovipositing more than
a month earlier than average. The first 7-spot matings were recorded on 26th
February in 1990, compared to an earliest of 21 April during the years 1985 to
1988 inclusive. The progeny of this early bout of reproduction hatched, and
because of the mildness of the winter, they found aphid food as the aphids had also
begun to reproduce earlier than usual Larval development was slower than usual
because of the lower average temperatures during March and April 1990,
compared to May and June in most years when larvae would normally be feeding
up. However, the first pupae were recorded in the last week of April 1990, and the
first newly eclosed adult appeared on 8th May. By this time, eggs and young larvae
were very plentiful, and conditions for their development appeared ideal as aphid
population densities were high.

So why did these good early signs not result in the expected population
explosion? There were several contributory factors. Some sharp late frosts may
have caused some mortality: ladybirds are known to be particularly susceptible to
hard early and late frosts. The dullest June, according to records at Heathrow
Airport Meteorological Station, since records began there in 1957, must also have
slowed down the reproductive rate of many ladybirds. But neither of these climatic
factors could account for the decline in numbers of ladybirds that occurred in late
June. Surveys at a number of sites in southern England in May and June showed
egg and larval densities to be very high and aphids to be plentiful stilL The density
of ladybird pupae on nettle-beds at Box Hill, Surrey, in late June, was the highest
we have ever seen, with almost every nettle leaf having at least one pupa on it. One
leaf had 13, and one single nettle stem had some 126 final instar larvae, pre-pupae,
pupae and newly emerged adults upon it. One 550 square metre nettle-bed, in the
grounds of Juniper Hall, near Dorking, Surrey supported a population of close on
one hundred thousand 7-spots, 2-spots, 10-spots and 14-spots. By this time there
were already indications of some cannibalism among the ladybirds as aphid
populations began to decline under the burden of predation. But the main reason
for the fact that excessive numbers of adult ladybirds never materialized is that the
vast majority of pupae never hatched. They were hit by one main predator and two
species of parasitoid.

The predator was the bug Deraeocoris ruber (L.). Both the nymphs and adults of
this bug are predatory, usually feeding on a variety of types of prey. However, in
June 1990 they appeared to be specializing on ladybird pupae, probably as a result of
the abundance of the pupae and their relative defencelessness against this predator.
While it is impossible to give an accurate estimate of the mortality level of ladybird
pupae due to attacks by this bug, because of the difficulty of detecting attack marks,
the level in June 1990 was certainly over 10% and may have been much higher.

The two parasitoids were the phorid flies Phahcrotophora fasciata (Fall) and P,
heroUnemis Schmitz. These flies attack and kill ladybird pre-pupae and pupae. In
1990, about 60% of 2-spots and 75% of 7-spots were attacked (Disney et aL, 1994).
These are the highest parasitoid rates of ladybird pupae by phorids that have been

BR. J. ENT. NAT. HIST., 9: 1996

recorded anywhere. The reason that the rate was so high in 1990 appears to be that
the progeny of the ladybirds that bred early (in February, March and April)
provided the phorids with hosts early in the year, so that they were able to produce
an extra generation. The phorids that developed inside ladybird pupae in June/July
in such large numbers were the grand-children of those that had overwintered.

A staggered reproductive period thus may allow parasitoids of ladybirds to
increase their numbers by increasing the number of generations, to the eventual
detriment of their hosts.

Other predators, including later-developing ladybird larvae and lacewing larvae
that were running short of aphid food, undoubtedly accounted for still more pupae.
In the end, given its early promise, 1990 ended up as a very disappointing year for
ladybirds.

To our list of conditions for ladybird population explosions, we must then add
that predators and parasitoids of ladybirds must not be abnormally common.

Which species of ladybirds are prone to population explosions?

The ladybird that is most often recorded in swarms or plagues in Britain is the 7-
spot ladybird. This is partly because it is a large and obvious species, and so large
increases in its numbers are very noticeable. The 7-spot is also the most abundant
British species of ladybird, and so starts from a higher population density base when
it does increase in number rapidly. However, the 7-spot is not the only British
ladybird that exhibits very rapid population increases. All the aphidophagous species
of ladybird that do not exhibit strong host-plant preferences may increase in number
spectacularly over a single reproductive season. These species include the 7-spot, 2-
spot, lO-spot, 11-spot and 14-spot ladybirds, the cream-spot and Adonis' ladybird
Adonia variegata (Goeze). All these species exhibit great fluctuations in numbers,
although the increases in numbers of Adonis' ladybird and the cream-spot ladybird
are rarely noticed as their usual densities are relatively low. All seven of these
species lay eggs in clutches on a wide variety of host plants, as long as suitable
aphid prey is available. This means that these species can respond to rapid
increases in aphid numbers. Furthermore, they can also respond to declines in
aphid colonies, which are notoriously ephemeral, by flying to other plant species to
seek alternative aphids.

It is pertinent not only to say which species do show large-scale changes in
population size, but also to explain why other species do not. In the case of the
four species of British ladybird that are not predatory, the explanation of their
relative stability in population size is simply a consequence of a relatively
consistent food supply. These species, the mildew-feeding 16-spot Tytthaspis
sedecimpunctata (L.), 22-spot Thea vigintiduopimctata (L.) and orange Halyzia
sedecimguiiata (L/) ladybirds and the leaf-eating 24-spot ladybird SubcoccineUa
vigintiquattuorpunctata (L.), are much less subject to the vagaries of the weather
than the predatory species. For example, we have monitored orange ladybirds at
Box Hill each year since 1987. The timing of the first reported mating of the year
has only varied by eight days (9-16 June) over nine summers (1987-1995).
Furthermore, the rate of larval development in these species is somewhat slower
than that of the aphid-eating species, presumably because mildews and leaves are
less nutritious than aphids.

The three British coccid-feeding ladybirds, the heather ladybird Chilocorus
bipustulatus (L.), the kidney-spot ladybird Chilocorus renipustidatus (Scriba), and
the pine ladybird Exochomus quadripustulatus (L.), do not produce massive

BR. J. ENT. NAT. HIST., 9: 1996

population explosions. This is probably a consequence of three factors. First, their
preferred prey, coccids (scale insects) do not increase their numbers as fast as do
many species of aphid. Second, there are less species of coccid than aphid, so that
should one species of coccid-feeding ladybird increase in number so substantially
that it ate out one species of coccid, it would have less in the way of potential
alternative types of principal prey (prey that promotes oviposition and allows full
larval development, Hodek, 1973), Third, and perhaps most significantly, these
three species of ladybird all lay their eggs in batches of just one to three eggs
underneath coccids, or adelgids in the case of the pine ladybird, or occasionally in
bark crevices close to them. Ovipositing in this way, these species do not have the
scope for reacting to particularly favourable conditions as fast as some of the
aphid-feeders which may lay batches of several dozen, or in the case of the 7 -spot ,
over a hundred eggs in a single clutch if conditions are right.

Aphidophagous host-plant-speciahst ladybirds also rarely produce massive
population explosions on a wide scale. Here the reason is again probably related to
a lack of a range of different principal foods that would allow these species to
maintain a high reproductive output once one prey species began to decline as a result
of increased ladybird numbers. In addition, several of the host-plant specialists seem
to show obligate univoltinism. For example, in Britain, both the eyed Anatis oceUata
(L,) and striped Mysia oblongoguttata (L,) ladybirds apparently have a requirement of
passing through a dormant period during the winter before they will begin to
reproduce.

Having said that species other than the aphidophagous generalists do not produce
population explosions on a wide scale, we have had records of local explosions for
several of these species. These records include abnormally high numbers of larch
ladybirds Aphidecta ohUterata (L,) in conifer plantations in Suffolk and Perthshire
during 1986 and 1989 respectively; very large numbers of pine ladybirds on Scots
pines, at Lakenheath Warren and the King's Forest, Suffolk, in both 1985 and 1989;
and exceptional numbers of hieroglyphic ladybirds CoccineUa hieroglyphica L, at
Chobham Common in 1985 and 1989. In each case the increase in numbers was
probably a consequence of abnormally high local prey densities. In the cases of the
increases in larch ladybirds and pine ladybirds, we know this to be the case, high
numbers of adelgids being reported on conifers in each instance. In the case of the
increases in hieroglyphic ladybirds, we assume that the heather leaf-beetle Lochmaea
suturaUs (Thomson, C. G,), whose larvae appear to be the main food of the
hieroglyphic ladybird, must have been having bumper years, although, as we had not
at that time recognized the association between these two beetles, we were not
monitoring heather leaf-beetle numbers in 1985 or 1989.

Conclusion

Dramatic increases in ladybird population numbers are essentially the product of
the interaction of three groups of organism: aphids, ladybirds, and the predators,
parasitoids and parasites of ladybirds. These interactions are in turn dependent on
climatic factors. Relatively minor fluctuations in climate, or in the interactions
between these groups at one time of year, can have a greatly amplified effect later in
the season. This means that predicting ladybird population explosions is very
uncertain, as we found in 1990, However, the past history of reported explosions
suggests that, although unpredictable, these events do occur with surprising
regularity. For example, if 1996 does not produce a population explosion, the
period 1977-1996 will be the longest period w^ithout an explosion this century.

72 BR. J. ENX NAT, HIST., 9: 19%

Acknowledgements

We wish to acknowledge all the recorders of the Cambridge Ladybird Survey, and
members of the local and national media who sent in records of ladybird swarms, or
brought such incidences to our attention in other ways.

References

Disney, R. H. L., Majerus, M. E. N. & Walpole, M. 1994. Parasitic behaviour of
PhalacroUiphom fasciata and P. herolinensis (Diptera: PhoridaeJ on coccinellids.
Entomologist 113: 28—42.

Hemptinne, J.-L., Dixon, A. F. G. & Coffin, J. !992. Attack strategy of ladybird beetles
(CoccincUidae): factors shaping their numerical response. Oevohgia 90: 238-245.

Hodek, L 1973. Biology of CoccineUidae. Junk, The Hague/ Academia, Prague.

Appendix 1.

Anecdotal accounts during the ladybird population explosion of 1976

''In 1976, on a hot summer's day. I had occasion to visit an area between Elvedon
and Barnham, about one mile south of Thetford, the exact spot being known as the
Gorse Trading Estate. 1 must have been exceptionally lucky, or unlucky, as one of
these 'swarms' had landed on the road passing through the estate, along which I had
to traveL 1 can remember very clearly seeing the whole road covered in thousands of
red ladybirds and the area involved must have been at least 1 8ft x 20ft.
Unfortunately, I had to pass over them/' (Jack Easter)

''In the summer of 1976, my two daughters, who were then 15 and 12 respectively,
had to curtail one of their usual summer activities, swimming at Ruislip Lido
(Middlesex), because of ladybirds. They did not like going into the water because the
surface was covered with millions and millions of ladybirds. The Lido has an
artificial beach for the bathers, and the sand was often also thick with ladybirds. I do
not know what type of ladybirds they were. Although my daughters are both grown
up now, and both have children of their own, I know they still remember 1976, as I
have heard them tell their children the story of when 'ladybirds stopped play'!" (Sally
Wheat ley)

"In 1976 we had a plague of ladybirds here in Leeds. They were all over the place,
on every pavement, and at times it was impossible to take a step without treading on
them/' (Frank Haiste)

"During the plague of 1976, while on holiday at Minster, in Sheppey, everywhere
one put one's foot, it was thick with ladybirds. The posts along the sea-front holding
up the chains were completely smothered.'' (Brenda Madgwick)

'T remember the ladybirds of 1976 well. I then lived on the Norfolk coast, and
used to take my dog for a walk along the sand each day. In 1976, for what must have
been about six weeks, we daily had the company of millions and millions of ladybirds
along the beach. The numbers appeared to fluctuate from day to day, but I guess the
zenith was about the end of July, or perhaps early August. The nujgibers on the beach
were quite extraordinary, and once the day got hot, many thousands were airborne.
Eventually, we began taking our walk early in the morning so as to avoid the
ladybirds when they were in flight. Remarkably, after this had been going on for
many weeks, one day the whole lot just disappeared. I have no idea why, because, as
far as 1 could see, the weather had not changed to any appreciable extent. One other

BR. J. ENT NAT. HIST., 9: 1996

thing may be of interest. For most of the period when the ladybirds were about, the
tide-line was soHd with the corpses of dead ladybirds, I presume that these
unfortunates had just drowned and been washed up." (John Trent)

''Beating trees and shrubs for moth larvae became a futile occupation in the latter
part of the summer in 1976. After just one or two taps, the beating tray contained
nothing but a seething mass of ladybirds. In Surrey, they were on almost every type
of tree, with birch, oak, and sallow perhaps being the worst. Several species were
present, Coccinella septempunctata was by far the most abundant, but several other
species were also very common. These included Adalia bipimctata^ AdaHa decern-
punctata^ Propylea quattuoydecimpunctata^ Coccinella imdecimpunctata and Calvia
quattuordecimgiittata which was particularly common on the birches. In over 25
years of moth collecting I cannot remember anything else like it. For several weeks, I
have entered in my notebook for 1976, 'Ladybirds stopped work!'/' (Stephen
Moore)

"In the summer of 1 976, on a humid and very hot night, I was on duty in the boiler
house of the Maelor Hospital, Wrexham. The air was heavy with greenfly. A light
breeze brought them in their millions causing them to sway one way, then the other.
Huge numbers were drawn into the one boiler that w^as on stream. With a long brush
I swept and shovelled them away throughout the night, until a dawn breeze wafted
them away. The next night, or the one following, the same conditions applied, but
this time it was the ladybirds that came, again in their millions, covering everything/'
(anon.)

'T have just heard you on the radio talking about ladybirds biting people when
they are starving hungry. I w^as most interested to hear what you had to say, because
I can confirm every word from personal experience. It was one year, back in the
seventies, and from what you say, it sounds like 1976, in the school holidays. I had
three young boys to amuse for six weeks, and because it was such a nice summer,
every day seemed to be sunny, I promised the boys that I would take them to the sea-
side. 1 guess the first attempt was about the middle of July. We went off to Brighton,
getting to the beach about 1 1 o'clock. By ten minutes later we were back in the car.
Both Robert, my youngest, and I had several little bumps on our arms and legs from
bites from ladybirds. They were absolutely everywhere, and thousands were in the
air. They landed on us all the time, and when we got back into the car, we had to
spend quite a while taking them off each other and out of our hair.

It was quite possible to feel when one bit us. It was a shght sort of pricking
sensation, not much more than a light tickle, but after a minute or two, it began to
sting and itch, so that it was quite unpleasant. We tried to do a few things around
Brighton, away from the sea-front, which was pretty unpopulated because of the
ladybird hordes, and ended up going to the pictures in the afternoon. Really a rather
abortive day, but I do not think I have ever seen so many of one kind of animal
together as I did that day.

In the next week, still trying to keep my promise to the boys, we tried again twice^
once going to a spot near Hastings, and then trying the east coast at Frinton, but it
was more or less the same story each time. In the end we gave up. I still like ladybirds
as much as ever, but had long resented the fact that these objects of my aftections
had turned on me that year. Now that I know^ from your broadcast that they were
starving to death, I can forgive those that took a bite out of me, and indeed, I feel
rather sorry for them. It's a shame that they did not get anything nutritious out of
me,'' (Angela Snow)

BR. J. ENT NAT. HIST., 9; 1996

'The ladybird swarms of 1976 were not confined to the land. On occasion^ my
daughter and her husband reported sailing through seas that were completely
covered in ladybirds." (Bridget Chadwick)

"On 11 July 1976, I was piloting a light aircraft about 20 miles south of
Manchester at 1500 ft when I flew into a large swarm of ladybirds. It was like flying
into bird shot. I put down at East Midlands airport to clean the canopy and check
the air intakes. There were hundreds of the little creatures, alive, crawling all over the
plane." (Signature illegible)

Appendix 2.

Some observations of swarms or exceptional numbers of ladybirds

recorded during the cambridge ladybird survey

11 June 1984, ''Very large numbers of many species of ladybird on trees and
shrubs near Santon Downham. Most were on broom, maple and Scots pine. Species
included: Anatis oceUata, Exochomus quadripustidatus^ Adalia hipunctata, Coccinella
septempunclata, Adalia decempimctala^ Calvia quattuordecimguttata and Harmonia
quadripimctala, in descending order of abundance. Aphids were very abundant on
the trees and many of the ladybirds were mating." (Peter Kearns and Simon
Albrecht)

February 1985. ''On Lakenheath Warren, exceptional numbers of pine ladybirds
Exochomus quadripustidaius sunning themselves on pine trees. The majority were
sitting still on pine cones in the sun. Some pine cones had as many as 30 ladybirds on
them. Everywhere we looked, the pine trees had these ladybirds. As we walked for
several miles through pine woods, the number of pine ladybirds must have run into
many millions." (Heather Ireland)

May 1987. "Along the banks of the River Aire, West Yorkshire, thousands and
thousands of ladybirds on the undergrowth and particularly on nettles." (Hazel
Dunning)

Mid-May 1989. "A large drift of ladybirds appeared on a beach at Bude,
CornwalL" (Ian CobbledichJ

June 1989. "On the ferry from Harwich to Esjberg, Denmark, the boat was
covered in thousands of ladybirds. Many of them were dead." (Mrs G. Stoller)

4 August 1989. '^Between 15.30 and 16.15 hrs a mass migration of CoccineUa
septempmictata took place, flying from east to west at altitudes from ground level
up to 50-60 ft at Little Stambridge Hall, near Rochford, Essex (TQ887919).
Estimated to have involved many hundreds/' (Observer — ^L. Watts, reported by
Roger Payne)

An extract from the 1989 Annual report of the Norfolk Ornithologists Association,
communication by Alan Paine read as follows, ''Ladybird: swarms on 20 July, flying
east from 1 1 a.m, to 1 p,m, and again later in the afternoon".

Three years later I received a pair of hnked reports from the same source. The 1992
Norfolk Ornithologists Association Report, again sent in by Alan Paine, described
events that took place at Holme, Norfolk. '^One of the largest swarms of hoverflies
that I have ever seen started to arrive on 31 July and^ continued the next day until
1.30 p.m., in ever-increasing numbers, all flying non-stop down a south-westerly
wind. Together with ladybirds, the mixed hordes meant a retreat to the HBO Centre

BR. J, ENT. NAT. HIST., 9: 1996

with door and windows firmly shut! 'Hundreds' even reached North Sea oil-rigs on
16 August and again 12 September,"

And later^ "Large swarms of 7-spot ladybird downwind (with hoverflies) on 31
July reaching a peak next day between 2 and 4.30 p.m., when flower spikes of
marram grass were festooned. A few continued to arrive on 2 August and there were
swarms everywhere. Some were so hungry they were eating hoverflies and their dead
companions'',

"The summer of 1989 was spectacular for ladybirds at Bettystown, near the
mouth of the river Boyne, on the east coast of Ireland. There were hterally
thousands to be seen each day on the local golf course, and nobody locally could
remember a more prolific year. In the memories of my children, 1989 will always be
referred to as the ladybird summer."*' (Hugh Leech)

In the spring of 1990, we received reports of exceptional numbers of ladybirds
from many parts of Britain. To give just a few examples; F. M. Unsworth reported
from Hexham, Northumberland, ''large numbers of ladybirds on open peat of
previously burned moorland''; H. Bremner, from Biggin Hill, Kent, ''thousands of
ladybirds in my garden"; L. Owen, from Kirriemuir, Angus, noted "a great many
ladybirds, all 7-spots, on our lawn"; C. Hurcombe, from Caversham, Berks., *T have
a plague of ladybirds in my garden''; from Stamford Bridge, H, Goodwin reported
'large numbers in garden, rose stems encrusted with the beetles, clusters of ladybirds
hidden just underneath the topsoil throughout the front and back gardens"; and
from Catherine Brown in Wigan, 'last weekend thousands of ladybirds were basking
in the sunshine of our south facing garden",

17 March 1990, ''While on a walk on the North York Moors, I came across a
swarm of ladybirds. They were in dense clusters all over the ground between low
stunted heather bushes {Calhma vulgaris) no more than 4 ins in height." (J, Salter)

17 April 1990. ''Astounding numbers of ladybirds all over the garden and
surrounding area (alt. 750 feet) this spring. Location: Far-Ben, Dunsmore,
Wendover, Bucks." (V. Piery)

Late July 1990. "A large plague of ladybirds, mainly 7-spots, at Weston-
Super-Mare." (P. Lenin). This swarm was noteworthy enough to be mentioned
in the Daily MaiL

Summer 199L "Noticed 1000+ 7-spot ladybirds gathering on a concrete wall
adjacent to the River Thames at Cliffe Marshes, Kent. Many were mating. A kestrel
was sitting on the wall, possibly eating ladybirds." (Dr L. Love)

8.00p.m., 7 August 199L "Rushen Gout, North of Aust (ST582906); many
thousands of 7-spots on the nationally scarce grass Alopecurm bulhosm on
grazed saltmarsh beside the River Severn." (M. Kitchen)

21 April 1992. At Leigh (TQ5646): "Literally hundreds of 2-spot ladybirds in every
crack and crevice on a telegraph pole, in the middle of a field. They appeared from
about one foot off the ground, to a height as far as I could observe. One week later
they had all disappeared/' (Vic Measday)

Several recorders sent me the same cutting from the Eastern Daily Press of 28 July
1994. Under the headline '"Basher barnee bee army breezes in, Holidaymakers took
to their heels yesterday as a red army blew into town. The Norfolk seaside town of
Wells was so over-run by an invasion of ladybirds that tourists and locals were
driven inside to escape. In places the town looked prepared for a royal visit — with an

unblemished carpet of red bishee barnee bees (as ladybirds are called locally)
covering the roads and pavements. Barry Franklin, from Derby, said: 'I've never
seen anything like it. They're all over the place, you just can*t escape them. I don't
usually mind ladybirds, but this is just making me uncomfortible\ And Jane Hood,
from Clacton, said: 'My two young children are sitting in the car because they're
frightened, and we're about to get away from Wells\"

Late July-early August 1994. "2=3 mile stretch on the coast between Heacham and
Hunstanton. Thousands flying everywhere. On a 16-metre long section of sea wallj
13000, mainly 7-spots, were counted." (Richard Rockcliffe)

5 August 1994. At Hunstanton: "I parked my car near the Old Lighthouse. On the
ground were very many stationary 7-spot ladybirds (20-40 per square foot of turf or
tarmac). Later, walking back towards town, I found ladybirds as thick as chipping
on a newly dressed road. There could have been a million," (H. Shelton)

From The Bristol Evening Post, 9 August 1994. "A village near Bristol has been
invaded by a swarm of ladybirds. The streets of Severn Beach are running red with
tens of thousands of the insects. Villagers said it is the largest invasion since the
drought year in 1976/'

Alan Paine reported reading, on the board of sightings at the Landguard Nature
Reserve/Bird Observatory for 14 August 1994: "Huge arrival of ladybirds". No
indication of numbers or species was given.

In addition we received these reports of events before the Survey.

"I remember visiting our small coastal town of Southwold when I was five years
old (1964), and the car, us, and all in this one spot by the sea were covered with
ladybirds." (Carmela Robinson).

May/June 1982. ''While on holiday in France, at Sables d'Or, Brittany, we saw at
the edge of the sea, coming in on the tide and walking up the beach, rows and rows of
ladybirds. They were on the water being washed in, and many had managed to walk
quite a distance up the beach (opposite to the lemming syndrome)/' (Gillian Siddy)

SHORT COMMUNICATION

Leiophora {Arrhinomyia) innoxia (IVleigen) (Diptera: Tachinidae) parasitizing the
ground-hopper Tetrix undulata (Sower by) (Orthoptera: Tctrigidae)* — I collected an
adult female Tetrix undulata (Sower by) and a larva on Arbrook Common, Esher,
Surrey on 6.V.1992 and retained them isolated in individual containers with a view to
photography. The female died c. 10.v and a dipterous puparium was noted in the
container shortly afterwards. A male Leiophora innoxia Meigen emerged 25. v. 1992.
No exit hole was visible; the neck membrane was intact. Presumably the parasite
emerged from under the pronotal extension above the abdomen. The similarity of
this structure to a beetle's elytra may have some bearing on why a parasitoid
previously recorded from the flea beetles Hahicus should attack a ground-hopper.^ —
R. W. J. Uffen, 4 Mardley Avenue, Welwyn, Hertfordshire AL6 OUD,

The following text is generated from uncorrected OCR.

[Begin Page: Text]

'^ APRIL 1996

ISSN 0952-7583

Vol. 9, Part 2

BRITISH JOURNAL OF

ENTOMOLOGY

AND NATURAL HISTORY

'^ TOA %

[Begin Page: Text]

BRITISH JOURNAL OF ENTOMOLOGY AND NATURAL HISTORY

Published by the British Entomological and Natural History Society

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B. Goater, B.Sc, M.LBiol. P. A. Sokoloff, M.Sc, C.Biol., M.L Biol., F.R.E.S.

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[Begin Page: Page 65]

BR. J. ENT. NAT. HIST., 9: 1996 65

LADYBIRD POPULATION EXPLOSIONS

Michael E. N. Majerus and Tamsin M. O. Majerus

Department of Genetics, University of Cambridge, Downing Street, Cambridge CB2 3 EH.

Tiiis paper lias been prompted by two recurring features of tine iast ten years,
during wliicli time we iiave co-ordinated a nation-wide survey of iadybirds in Britain.
One invoives tine pinone caiis we receive from members of tlie media, aimost every
year in iate Juiy, or eariy August, asking us to comment on 'piagues of iadybirds'
tiiat iiave liit such-and-sucli a piace. Tine second invoives tlie commonest iadybird
reminiscence of members of tine pubiic wliom we Iiave met wliiie traveiiing around
Britain eitlier looking for, or talking about ladybirds: "the year that ladybirds went
mad". Almost all are remembering the last 'great ladybird year' — 1 976; although a
few also recall the preceding one, in 1959. Much has been written in the press, both
local and national, about plagues of ladybirds, their causes and effects. A
considerable proportion of these column inches is ill-informed. The impression we
often get when talking to journalists is that their aim is to write something that is
worth printing, even if it means making a proverbial mountain out of a molehill. In
our opinion this is something of a shame, as regular and often exaggerated reports of
natural phenomena, which are a little out of the ordinary, undoubtedly devalue
reports of truly extraordinary natural phenomena. Ladybird 'plagues' provide an
excellent example, for major ladybird population explosions are truly amazing
events.

Ladybird plagues, whether widespread or local, are well-known natural
phenomena. Records of swarms are scattered widely through the literature over
the past century. In this paper we give an account of some of the reminiscences of the
most recent major ladybird population explosion in Britain, that of 1 976, put on
record some of the sightings of ladybird swarms between 1984 and 1995, and
comment on the causes and consequences of ladybird swarms.

THE 'GREAT LADYBIRD YEAR' — 1 976

The population explosion of ladybirds in 1976 was truly astounding. It extended
across more or less the whole of England and Wales, and into some parts of southern
Scotland. Exceptional numbers of ladybirds were also recorded in many parts of
north-west Europe. The huge numbers of ladybirds were widely reported in
newspapers, and on television and radio. In Appendix 1 are listed a small sample of
the many reports and reminiscences of the plagues of ladybirds that year.

The summers of both 1 975 and 1 976 were unusually warm a nd sun ny. Because the
extent of any change in the size of a population of organisms»t|'^g'^svT'^sQme extent

e-mail: menm@mole.bio.cam.ac.uk MAY 1 1QQ6

OftR ARIES

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66 BR. J. ENT. NAT. HIST., 9: 1996

on the base number of individuals, it is pertinent to start in the summer of 1 975. This
was a long warm summer, not exceptionally dry or hot, but with warmer
temperatures than average and no significant long periods of bad weather. Both
aphids, and the ladybirds that fed on them, did well. At least some species of ladybird
such as the 2-spot Adalia bipanctata (L.), 7-spot Coccinella septempunctata L. and
10-spot Adalia decempunctata (L.) produced a partial second generation.
Consequently, by the autumn of 1975, the populations of aphidophagous ladybirds
that retired to their overwintering refuges were already larger than usual. The
ensuing winter was remarkably mild, with a result that the winter mortality rate of
ladybirds, which is usually in excess of 50%, was much lower than this figure,
thereby increasing the number of ladybirds in the spring of 1 976 still further,
relative to the norm.

The spring of 1 976 was not exceptional. If anything it was wetter and warmer than
average. Aphid populations did well, reproducing rapidly on the lush plant growth,
so that when ladybirds began to venture out in search of food, they found plentiful
supplies. Cloudy weather in April and early May prevented most species from
indulging in much mating early on, but when the sunny weather began in the middle
of May, conditions were ideal. The ladybirds began to mate and oviposit at a very
rapid rate. Larvae found plentiful supplies of aphids and, as temperatures rose to
record levels, they fed up and completed their development in an abnormally short
time for Britain. The hot sunny weather continued day after day through the rest of
May and throughout June. By the end of June, the extent of the increase in ladybird
numbers, in England, Wales and southern parts of Scotland, was already evident.
They were everywhere. The species that showed the greatest increases were the
aphidophagous generalists, such as the 7-spot, 2-spot and 14-spot Propylea
quattuordecimpunctata (L.), but most other species did better than usual and 10-

spot, 11 -spot Coccinella undecimpunctata L. and cream-spot ladybird Calvia
quattuordecimguttata (L.) were more abundant than we have seen them since.

This increase in the number of adult ladybirds, combined with the huge densities
of ladybird larvae among aphid colonies (and high densities of other aphid predators
and parasitoids) led to a dramatic crash in aphid numbers. Bluntly, the aphids were
massacred. Even with their phenomenal reproductive rate, the aphids could not keep
up with the losses due to predation. Worse still, the plants that the aphids were
feeding upon were deteriorating rapidly under the scorching heat of that long hot
summer.

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BR. J. ENT. NAT. HIST., 9: 1996 67

We once calculated the approximate number of ladybirds that would have been in
the tide-lines along the southern and eastern coasts of Britain on a single day in late
July 1 976. Assuming all to have been one of our larger species, the 7-spot (which they
were not), a conservative estimate gives a figure of some 23 654400000 ladybirds
(estimated by counting the number of 7-spots in 1 5 200 mm sections of tideline and
multiplied up by the amount of suitable coastline between Land's End and the east
coast border between England and Scotland). This figure is difficult to comprehend,
but it is about four times the current human population of the Earth and, of course,
this was just the ladybirds in the tide-lines on a single day. It does not include any of
those that stayed on land, or those that were washed out to sea, or those that were
eaten by other starving ladybirds or other predators, or those that were killed on the
roads or elsewhere by the devices of man. Not surprisingly, a crash in ladybird
populations followed.

From this description, it may be seen that a whole sequence of conditions are

involved in the run up to one of tine great iadybird expiosions: favourable
conditions tine previous year, a miid winter, plenty of food in the spring, a single
and synchronized start to the reproductive season brought on by a sharp
improvement in sunshine hours, and hot and sunny weather in the summer to
promote mating, egg laying and rapid larval development. It is not often that all
these conditions fall together, with the result that widespread ladybird explosions
are rare in Britain, occurring, on average, about once every 15 years. We seem to
be overdue for one now.

Records of ladybird plagues' in Britain: 1984-1994

The reasons that large numbers of ladybirds tend to be observed together in these
two periods are different. Many species of ladybird aggregate together to pass the
winter. Large numbers of individuals may find the same sheltered area out of the
w orst of the winter weather. When these first venture out from their overwintering
sites they will often do so en masse. On bright spring days, ladybirds will often sun
themselves to warm up. This means that large numbers will be seen in exposed
positions sunning themselves before moving off to forage for food. Most of these
spring records involve the 7-spot ladybird which tends to overwinter close to the
ground, or occasionally underground, often in large aggregations. On leaving its
overwintering sites, this species will first climb low spring vegetation to sun itself, to
drink dew or rain drops to replenish its fluid reserves, and to look for whatever food

it can find. Here, on low-growing green leaves, it is very obvious and easily seen.

It is in high summer that the largest numbers of ladybirds come together and
sometimes form what may realistically be called swarms. At this time of year, the

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68 BR. J. ENT. NAT. HIST., 9: 1996

adult population usually reaches its maximum. The new generation of adults has not
yet begun to diminish because of the perils of life: lack of food, predators, parasites,
disease, adverse climate or pure accident. The main activity for these young adults is
to feed up for the winter. In many years, aphid populations "decline dramatically in
high summer, partly as a result of predation pressure and partly because their host
plants are deteriorating. The consequence is that in areas where food has become
scarce, ladybirds will often take to the air on hot summer days, to seek food
elsewhere. The direction of ladybird flight is not entirely active. While ladybirds can
fly in a direct way, when they take to the air on hot days their flight is aided by
thermals. These thermals may take the ladybirds up hundreds, even thousands, of
metres into the sky. At high altitude, they are then blown by the prevailing winds,
often for many miles. Due to the nature of thermals in some places, and particularly
along the coast, large numbers of these high-flying ladybirds may be brought back to
earth in the same place, producing very high population concentrations. The
prevailing south-westerly winds are thus responsible for the fact that swarms are
more often recorded from the east coast of England than any other region.

The necessity for synchronized reproduction

Most of the factors that appear to be prerequisites for iadybird popuiation
expiosions are easiiy understood. Good ciimatic conditions and high food
avaiiabiiity the previous year lead to an increase in the number of ladybirds that
feed up well prior to overwintering. The high fat reserves of these ladybirds, and/or a
mild winter, will result in reduced mortality during this critical period. Conditions
under which aphids flourish in the spring will lead to good supplies of food for the
ladybirds when they leave their overwinter sites and begin feeding and reproducing.
Warm sunny weather in late spring and early summer will induce high mating
frequencies and rapid oviposition. The high aphid density and warm weather will
allow rapid larval development and reduce the level of larval cannibalism. All this is
intuitively sensible. However, the reason why a single and synchronized start to the
ladybird reproductive season is crucial, is not so obvious. It is not easy to see why a
somewhat staggered start to the reproductive season would not produce even greater
numbers if the start of the season were extended earlier. Surely, one might expect that
a staggered start would produce greater numbers because, at least in the early part of
the season when aphid numbers are just building up, survival of larvae would not be
reduced too much by competition between them.

Three factors appear to be important in reducing the benefit of an early and
staggered start to the ladybird reproductive season. First, any significant predation
early in the spring may greatly reduce the eventual numbers of aphids produced
later in the year. Because aphids have such a phenomenal reproductive potential
within a single year, the death of a single female in April may reduce the July
population by many millions. Second, as female ladybirds are reluctant to oviposit
on plants which are already inhabited by ladybird larvae (Hemptinne et al., 1992),
stretching the reproductive season will reduce the number of suitable oviposition
sites for females later in the season. Third, and perhaps most crucially, parasitoids

of ladybirds may benefit from tine presence of immature stages over a protracted
period. Monitoring of population demograpliy in 1990 sliowed tine influence that
predators and parasitoids may have in years when the reproductive period for
ladybirds is staggered.

Records in late 1989 indicated that numbers of several of the generalist species of
ladybird were at their highest levels since 1 976. The winter of 1 989/90 was

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BR. J. ENT. NAT. HIST.. 9: 1996 69

because of the lower average temperatures during March and April 1990,
compared to May and June in most years when larvae would normally be feeding
up. However, the first pupae were recorded in the last week of April 1 990, and the
first newly eclosed adult appeared on 8th May. By this time, eggs and young larvae
were very plentiful, and conditions for their development appeared ideal as aphid
population densities were high.

So why did these good early signs not result in the expected population
explosion? There were several contributory factors. Some sharp late frosts may
have caused some mortality: ladybirds are known to be particularly susceptible to
hard early and late frosts. The dullest June, according to records at Heathrow
Airport Meteorological Station, since records began there in 1957, must also have
slowed down the reproductive rate of many ladybirds. But neither of these climatic
factors could account for the decline in numbers of ladybirds that occurred in late
June. Surveys at a number of sites in southern England in May and June showed
egg and larval densities to be very high and aphids to be plentiful still. The density
of ladybird pupae on nettle-beds at Box Hill, Surrey, in late June, was the highest
we have ever seen, with almost every nettle leaf having at least one pupa on it. One
leaf had 13, and one single nettle stem had some 126 final instar larvae, pre-pupae,
pupae and newly emerged adults upon it. One 550 square metre nettle-bed, in the
grounds of Juniper Hall, near Dorking, Surrey supported a population of close on
one hundred thousand 7-spots, 2-spots, 10-spots and 14-spots. By this time there
were already indications of some cannibalism among the ladybirds as aphid
populations began to decline under the burden of predation. But the main reason
for the fact that excessive numbers of adult ladybirds never materialized is that the
vast majority of pupae never hatched. They were hit by one main predator and two
species of parasitoid.

The predator was the bug Deraeocoris ruber (L.). Both the nymphs and adults of

this bug are predatory, usually feeding on a variety of types of prey. However, in
June 1 990 they appeared to be specializing on ladybird pupae, probably as a result of
the abundance of the pupae and their relative defencelessness against this predator.
While it is impossible to give an accurate estimate of the mortality level of ladybird
pupae due to attacks by this bug, because of the difficulty of detecting attack marks,
the level in June 1990 was certainly over 10% and may have been much higher.

The two parasitoids were the phorid flies Phalacrotophora fasciata (Fall.) and P.
berolinensis Schmitz. These flies attack and kill ladybird pre-pupae and pupae. In
1 990, about 60% of 2-spots and 75% of 7-spots were attacked (Disney et cd., 1 994).
These are the highest parasitoid rates of ladybird pupae by phorids that have been

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70 BR. J. ENT. NAT. HIST., 9: 1996

A staggered reproductive period thus may allow parasitoids of ladybirds to
increase their numbers by increasing the number of generations, to the eventual
detriment of their hosts.

Other predators, including later-developing ladybird larvae and lacewing larvae

that were running short of aphid food, undoubtedly accounted for still more pupae.
In the end, given its early promise, 1990 ended up as a very disappointing year for
ladybirds.

To our list of conditions for ladybird population explosions, we must then add
that predators and parasitoids of ladybirds must not be abnormally common.

Which species of ladybirds are prone to population explosions?

The ladybird that is most often recorded in swarms or plagues in Britain is the 7-
spot ladybird. This is partly because it is a large and obvious species, and so large
increases in its numbers are very noticeable. The 7-spot is also the most abundant
British species of ladybird, and so starts from a higher population density base when
it does increase in number rapidly. However, the 7-spot is not the only British
ladybird that exhibits very rapid population increases. All the aphidophagous species
of ladybird that do not exhibit strong host-plant preferences may increase in number
spectacularly over a single reproductive season. These species include the 7-spot, 2-
spot, 10-spot, 1 1 -spot and 14-spot ladybirds, the cream-spot and Adonis' ladybird
Adonia variegata (Goeze). All these species exhibit great fluctuations in numbers,
although the increases in numbers of Adonis' ladybird and the cream-spot ladybird
are rarely noticed as their usual densities are relatively low. All seven of these
species lay eggs in clutches on a wide variety of host plants, as long as suitable
aphid prey is available. This means that these species can respond to rapid
increases in aphid numbers. Furthermore, they can also respond to declines in
aphid colonies, which are notoriously ephemeral, by flying to other plant species to
seek alternative aphids.

It is pertinent not only to say which species do show large-scale changes in

population size, but aiso to expiain why otiier species do not. In the case of the
four species of British ladybird that are not predatory, the explanation of their
relative stability in population size is simply a consequence of a relatively
consistent food supply. These species, the mildew-feeding 16-spot Tytthaspis
sedecimpunctata (L.), 22-spot Thea vigintiduopunctata (L.) and orange Halyzia
sedecimguttata (L.) ladybirds and the leaf-eating 24-spot ladybird Subcoccinella
vigintiquattuorpunctata (L.), are much less subject to the vagaries of the weather
than the predatory species. For example, we have monitored orange ladybirds at
Box Hill each year since 1 987. The timing of the first reported mating of the year
has only varied by eight days (9-1 6 June) over nine summers (1 987-1 995).
Furthermore, the rate of larval development in these species is somewhat slower
than that of the aphid-eating species, presumably because mildews and leaves are
less nutritious than aphids.

The three British coccid-feeding ladybirds, the heather ladybird Chilocorus
bipustulatus (L.), the kidney-spot ladybird Chilocorus renipustulatus (Scriba), and
the pine ladybird Exochomus quadripustulatus (L.), do not produce massive

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BR. J. ENT. NAT. HIST., 9: 1996 71

alternative types of principal prey (prey that promotes oviposition and allows full
larval development, Hodek, 1973). Third, and perhaps most significantly, these
three species of ladybird all lay their eggs in batches of just one to three eggs
underneath coccids, or adelgids in the case of the pine ladybird, or occasionally in
bark crevices close to them. Ovipositing in this way, these species do not have the
scope for reacting to particularly favourable conditions as fast as some of the
aphid-feeders which may lay batches of several dozen, or in the case of the 7-spot,
over a hundred eggs in a single clutch if conditions are right.

Aphidophagous host-plant-specialist ladybirds also rarely produce massive
population explosions on a wide scale. Here the reason is again probably related to
a lack of a range of different principal foods that would allow these species to
maintain a high reproductive output once one prey species began to decline as a result
of increased ladybird numbers. In addition, several of the host-plant specialists seem
to show obligate univoltinism. For example, in Britain, both the eyed Anatis ocellata
(L.) and striped Mysia oblongoguttata (L.) ladybirds apparently have a requirement of
passing through a dormant period during the winter before they will begin to
reproduce.

numbers of adelgids being reported on conifers in eacli instance. In tine case of tine
increases in Inieroglypliic ladybirds, we assume that the heather leaf-beetle Lochmaea
suturalis (Thomson, C. G.), whose larvae appear to be the main food of the
hieroglyphic ladybird, must have been having bumper years, although, as we had not
at that time recognized the association between these two beetles, we were not
monitoring heather leaf-beetle numbers in 1985 or 1989.

Conclusion

Dramatic increases in ladybird population numbers are essentially the product of
the interaction of three groups of organism: aphids, ladybirds, and the predators,
parasitoids and parasites of ladybirds. These interactions are in turn dependent on
climatic factors. Relatively minor fluctuations in climate, or in the interactions
between these groups at one time of year, can have a greatly amplified effect later in
the season. This means that predicting ladybird population explosions is very
uncertain, as we found in 1990. However, the past history of reported explosions
suggests that, although unpredictable, these events do occur with surprising
regularity. For example, if 1996 does not produce a population explosion, the
period 1977-1996 will be the longest period without an explosion this century.

[Begin Page: Page 72]

72 BR. J. ENT. NAT. HIST., 9: 1996

Acknowledgements

We wish to acknowledge all the recorders of the Cambridge Ladybird Survey, and

members of the local and national media who sent in records of ladybird swarms, or
brought such incidences to our attention in other ways.

References

Disney, R. H. L., Majerus, M. E. N. & Walpole, M. 1994. Parasitic behaviour of
Phalacrotophora fasciata and P. berolinensis (Diptera: Phoridae) on coccinellids.
Entomologist 113:28'^2.

Hemptinne, J.-L., Dixon, A. F. G. & Coffin, J. 1992. Attack strategy of ladybird beetles
(Coccinellidae): factors shaping their numerical response. Oecologia 90: 238-245.

Hodek, I. 1 973. Biology of Coccinellidae. Junk, The Hague/ Academia, Prague.

Appendix 1.

Anecdotal accounts during the ladybird population explosion of 1976

"In 1976, on a hot summer's day, I had occasion to visit an area between Elvedon
and Barnham, about one mile south of Thetford, the exact spot being known as the
Gorse Trading Estate. I must have been exceptionally lucky, or unlucky, as one of
these 'swarms' had landed on the road passing through the estate, along which I had
to travel. I can remember very clearly seeing the whole road covered in thousands of
red ladybirds and the area involved must have been at least 18ft x 20ft.
Unfortunately, I had to pass over them." (Jack Easter)

"In the summer of 1976, my two daughters, who were then 15 and 12 respectively,
had to curtail one of their usual summer activities, swimming at Ruislip Lido
(Middlesex), because of ladybirds. They did not like going into the water because the

surface was covered with millions and millions of ladybirds. The Lido has an
artificial beach for the bathers, and the sand was often also thick with ladybirds. I do
not know what type of ladybirds they were. Although my daughters are both grown
up now, and both have children of their own, I know they still remember 1 976, as I
have heard them tell their children the story of when ladybirds stopped playT' (Sally
Wheatley)

"In 1 976 we had a plague of ladybirds here in Leeds. They were all over the place,
on every pavement, and at times it was impossible to take a step without treading on
them." (Frank Haiste)

"I remember the ladybirds of 1976 well. I then lived on the Norfolk coast, and
used to take my dog for a walk along the sand each day. In 1 976, for what must have
been about six weeks, we daily had the company of millions and millions of ladybirds
along the beach. The numbers appeared to fluctuate from day to day, but I guess the
zenith was about the end of July, or perhaps early August. The numbers on the beach
were quite extraordinary, and once the day got hot, many thousands were airborne.
Eventually, we began taking our walk early in the morning so as to avoid the
ladybirds when they were in flight. Remarkably, after this had been going on for
many weeks, one day the whole lot just disappeared. I have no idea why, because, as
far as I could see, the weather had not changed to any appreciable extent. One other

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BR. J. ENT. NAT. HIST., 9: 1996 73

thing may be of interest. For most of tlie period winen tine ladybirds were about, the
tide-line was solid with the corpses of dead ladybirds. I presume that these
unfortunates had just drowned and been washed up." (John Trent)

"Beating trees and shrubs for moth larvae became a futile occupation in the latter
part of the summer in 1 976. After just one or two taps, the beating tray contained
nothing but a seething mass of ladybirds. In Surrey, they were on almost every type
of tree, with birch, oak, and sallow perhaps being the worst. Several species were
present. Coccinella septempunctata was by far the most abundant, but several other
species were also very common. These included Adalia bipunctata, Adalia decem-
punctata, Propylea quattuordecimpunctata, Coccinella undecimpunctata and Calvia
quattuordecimguttata which was particularly common on the birches. In over 25
years of moth collecting I cannot remember anything else like it. For several weeks, I
have entered in my notebook for 1976, 'Ladybirds stopped work!'." (Stephen
Moore)

"In the summer of 1976, on a humid and very hot night, I was on duty in the boiler
house of the Maelor Hospital, Wrexham. The air was heavy with greenfly. A light
breeze brought them in their millions causing them to sway one way, then the other.
Huge numbers were drawn into the one boiler that was on stream. With a long brush
I swept and shovelled them away throughout the night, until a dawn breeze wafted
them away. The next night, or the one following, the same conditions applied, but
this time it was the ladybirds that came, again in their millions, covering everything."
(anon.)

"I have just heard you on the radio talking about iadybirds biting peopie when
they are starving hungry. I was most interested to hear what you had to say, because
I can confirm every word from personal experience. It was one year, back in the
seventies, and from what you say, it sounds like 1976, in the school holidays. I had
three young boys to amuse for six weeks, and because it was such a nice summer,
every day seemed to be sunny, I promised the boys that I would take them to the sea-
side. I guess the first attempt was about the middle of July. We went off to Brighton,
getting to the beach about 1 1 o'clock. By ten minutes later we were back in the car.
Both Robert, my youngest, and I had several little bumps on our arms and legs from
bites from ladybirds. They were absolutely everywhere, and thousands were in the
air. They landed on us all the time, and when we got back into the car, we had to
spend quite a while taking them off each other and out of our hair.

It was quite possible to feel when one bit us. It was a slight sort of pricking
sensation, not much more than a light tickle, but after a minute or two, it began to
sting and itch, so that it was quite unpleasant. We tried to do a few things around
Brighton, away from the sea-front, which was pretty unpopulated because of the
ladybird hordes, and ended up going to the pictures in the afternoon. Really a rather
abortive day, but I do not think I have ever seen so many of one kind of animal
together as I did that day.

In the next week, still trying to keep my promise to the boys, we tried again twice,
once going to a spot near Hastings, and then trying the east coast at Frinton, but it
was more or less the same story each time. In the end we gave up. I still like ladybirds
as much as ever, but had long resented the fact that these objects of my affections
had turned on me that year. Now that I know from your broadcast that they were
starving to death, I can forgive those that took a bite out of me, and indeed, I feel
rather sorry for them. It's a shame that they did not get anything nutritious out of
me." (Angela Snow)

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74 BR. J. ENT. NAT. HIST., 9: 1996

"The ladybird swarms of 1976 were not confined to the land. On occasion, my
daughter and her husband reported sailing through seas that were completely
covered in ladybirds.' 1 (Bridget Chadwick)

"On 1 1 July 1 976, I was piloting a light aircraft about 20 miles south of
Manchester at 1 500 ft when I flew into a large swarm of ladybirds. It was like flying
into bird shot. I put down at East Midlands airport to clean the canopy and check
the air intakes. There were hundreds of the little creatures, alive, crawling all over the
plane." (Signature illegible)

Appendix 2.

Some observations of swarms or exceptional numbers of ladybirds

recorded during the Cambridge ladybird survey

1 1 June 1 984. "Very large numbers of many species of ladybird on trees and
shrubs near Santon Downham. Most were on broom, maple and Scots pine. Species
included: Anatis ocellata, Exochomus quadripustulatus, Adalia bipunctata, Coccinella
septempunctata, Adalia decempunctata, Calvia quattuordecimguttata and Harmonia
quadripunctata, in descending order of abundance. Aphids were very abundant on
the trees and many of the ladybirds were mating." (Peter Kearns and Simon

Albrecht)

February 1985. "On Lakenheath Warren, exceptional numbers of pine iadybirds
Exoclnomus quadripustuiatus sunning tliemselves on pine trees. Tine majority were
sitting still on pine cones in the sun. Some pine cones had as many as 30 ladybirds on
them. Everywhere we looked, the pine trees had these ladybirds. As we walked for
several miles through pine woods, the number of pine ladybirds must have run into
many millions." (Heather Ireland)

May 1 987. "Along the banks of the River Aire, West Yorkshire, thousands and
thousands of ladybirds on the undergrowth and particularly on nettles." (Hazel
Dunning)

Mid-May 1989. "A large drift of ladybirds appeared on a beach at Bude,
Cornwall." (Ian Cobbledich)

June 1989. "On the ferry from Harwich to Esjberg, Denmark, the boat was
covered in thousands of ladybirds. Many of them were dead." (Mrs G. Stoller)

4 August 1989. "Between 15.30 and 16.15 hrs a mass migration of Coccinella
septempunctata took place, flying from east to west at altitudes from ground level
up to 50-60 ft at Little Stambridge Hall, near Rochford, Essex (T0887919).
Estimated to have involved many hundreds." (Observer — L. Watts, reported by
Roger Payne)

An extract from the 1 989 Annual report of the Norfolk Ornithologists Association,
communication by Alan Paine read as follows. "Ladybird: swarms on 20 July, flying
east from 1 1 a.m. to 1 p.m. and again later in the afternoon".

Three years later I received a pair of linked reports from the same source. The 1992
Norfolk Ornithologists Association Report, again sent in by Alan Paine, described
events that took place at Holme, Norfolk. "One of the largest swarms of hoverflies
that I have ever seen started to arrive on 31 July and continued the next day until
1 .30 p.m., in ever-increasing numbers, all flying non-stop down a south-westerly
wind. Together with ladybirds, the mixed hordes meant a retreat to the HBO Centre

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BR. J. ENT. NAT. HIST., 9: 1996 75

with door and windows firmly shut! 'Hundreds' even reached North Sea oil-rigs on
1 6 August and again 1 2 September."

And later, "Large swarms of 7-spot ladybird downwind (with hoverflies) on 31
July reaching a peak next day between 2 and 4.30 p.m., when flower spikes of
marram grass were festooned. A few continued to arrive on 2 August and there were
swarms everywhere. Some were so hungry they were eating hoverfhes and their dead
companions".

"The summer of 1 989 was spectacular for ladybirds at Bettystown, near the
mouth of the river Boyne, on the east coast of Ireland. There were literally
thousands to be seen each day on the local golf course, and nobody locally could
remember a more prolific year. In the memories of my children, 1 989 will always be
referred to as the ladybird summer." (Hugh Leech)

In the spring of 1990, we received reports of exceptionai numbers of iadybirds
from many parts of Britain. To give just a few exampies: F. M. Unswortli reported
from Hexinam, Nortlnumberiand, "iarge numbers of iadybirds on open peat of
previously burned mooriand"; H. Bremner, from Biggin Hiii, Kent, "tliousands of
iadybirds in my garden"; L. Owen, from Kirriemuir, Angus, noted "a great many
iadybirds, aii 7-spots, on our iawn"; C. Hurcombe, from Caverslnam, Berks., "I liave
a piague of iadybirds in my garden"; from Stamford Bridge, H. Goodwin reported
"iarge numbers in garden, rose stems encrusted witli tine beetles, clusters of ladybirds
hidden just underneath the topsoil throughout the front and back gardens"; and
from Catherine Brown in Wigan, "last weekend thousands of ladybirds were basking
in the sunshine of our south facing garden".

1 7 March 1 990. "While on a walk on the North York Moors, I came across a
swarm of ladybirds. They were in dense clusters all over the ground between low
stunted heather bushes (Calluna vulgaris) no more than 4 ins in height." (J. Salter)

1 7 April 1 990. "Astounding numbers of ladybirds all over the garden and
surrounding area (alt. 750 feet) this spring. Location: Far-Ben, Dunsmore,

Wendover, Bucks." (V. Piery)

Late July 1990. "A large plague of ladybirds, mainly 7-spots, at Weston-
Super-Mare." (P. Lenin). This swarm was noteworthy enough to be mentioned
in the Daily Mail.

Summer 1 991 . "Noticed 1 000+ 7-spot ladybirds gathering on a concrete wall
adjacent to the River Thames at Cliffe Marshes, Kent. Many were mating. A kestrel
was sitting on the wall, possibly eating ladybirds." (Dr L. Love)

8.00 p.m., 7 August 1 991 . "Rushen Gout, North of Aust (ST582906): many
thousands of 7-spots on the nationally scarce grass Alopecurus bulbosus on
grazed saltmarsh beside the River Severn." (M. Kitchen)

Several recorders sent me the same cutting from the Eastern Daily Press of 28 July
1994. Under the headline "Basher barnee bee army breezes in. Holidaymakers took
to their heels yesterday as a red army blew into town. The Norfolk seaside town of
Wells was so over-run by an invasion of ladybirds that tourists and locals were
driven inside to escape. In places the town looked prepared for a royal visit — with an

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76 BR. J. ENT. NAT. HIST., 9: 1996

unblemished carpet of red bishee barnee bees (as ladybirds are called locally)
covering the roads and pavements. Barry Franklin, from Derby, said: Tve never
seen anything like it. They're all over the place, you just can't escape them. I don't
usually mind ladybirds, but this is just making me uncomfortable'. And Jane Hood,
from Clacton, said: 'My two young children are sitting in the car because they're
frightened, and we're about to get away from Wells'."

Late July-early August 1994. "2-3 mile stretch on the coast between Heacham and

Hunstanton. Thousands flying everywhere. On a 1 6-metre long section of sea wall,
13 000, mainly 7-spots, were counted." (Richard Rockcliffe)

5 August 1 994. At Hunstanton: "I parked my car near the Old Lighthouse. On the
ground were very many stationary 7-spot ladybirds (20-40 per square foot of turf or
tarmac). Later, walking back towards town, I found ladybirds as thick as chipping
on a newly dressed road. There could have been a million." (H. Shelton)

Alan Paine reported reading, on the board of sightings at the Landguard Nature
Reserve/Bird Observatory for 14 August 1994: "Huge arrival of ladybirds". No
indication of numbers or species was given.

In addition we received these reports of events before the Survey.

"I remember visiting our small coastal town of Southwold when I was five years
old (1 964), and the car, us, and all in this one spot by the sea were covered with
ladybirds." (Carmela Robinson).

May/June 1982. "While on holiday in France, at Sables d'Or, Brittany, we saw at
the edge of the sea, coming in on the tide and walking up the beach, rows and rows of
ladybirds. They were on the water being washed in, and many had managed to walk
quite a distance up the beach (opposite to the lemming syndrome)." (Gillian Siddy)

SHORT COMMUNICATION

Leiophora (Arrhinomyia) innoxia (Meigen) (Diptera: Tachinidae) parasitizing tine
ground-liopper Tetrix undulata (Sowerby) (Ortlioptera: Tetrigidae). — I collected an
adult female Tetrix undulata (Sowerby) and a larva on Arbrook Common, Esher,
Surrey on 6.V.1 992 and retained them isolated in individual containers with a view to
photography. The female died c. lO.v and a dipterous puparium was noted in the
container shortly afterwards. A male Leiophora innoxia Meigen emerged 25. V. 1992.
No exit hole was visible; the neck membrane was intact. Presumably the parasite
emerged from under the pronotal extension above the abdomen. The similarity of
this structure to a beetle's elytra may have some bearing on why a parasitoid
previously recorded from the flea beetles Halticus should attack a ground-hopper. —
R. W. J. Uffen, 4 Mardley Avenue, Welwyn, Hertfordshire AL6 OUD.

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